RFR: 8253350: Remove unimplemented SharedRuntime::clean_*_entry

[Coleen Phillimore]

On Fri, 18 Sep 2020 11:02:28 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Their existence seems to predate OpenJDK. There are no uses, no definitions.
> 
> Testing:
>   - [x] Linux x86_64 fastdebug build
>   - [x] Text searches for `clean_*_entry` in `src/hotspot`

Also appears trivial.

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Marked as reviewed by coleenp (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/245